Fluid flow control damper assembly

ABSTRACT

A fluid flow control damper assembly having a damper vane and a piston actuator is relatively easy to install when the outer surface of a duct is inaccessible. The piston actuator includes a powder metal piston, lacking an elastomeric (for example, rubber) seal, that is closely received within a glass cylinder. The glass cylinder has a fitting that closes one end of the cylinder and is connectable to a pneumatic control system. A drive rod is connected between the piston and the damper vane to move the damper vane between an open position and a closed position.

FIELD OF THE INVENTION

The invention relates generally to fluid flow control dampers and moreparticularly to a low pressure fluid operated fluid flow control damperassembly particularly useful in HVAC systems.

BACKGROUND OF THE INVENTION

Fluid flow control dampers are used in a wide variety of applications,including (heating, ventilation and air conditioning) HVAC systems. InHVAC systems, dampers are used to control the supply of conditioned airto various rooms or zones within a building. One such damper device is amechanical damper assembly including a short piece of duct in which adamper vane is pivotally mounted by a shaft. The damper vane isrotatable between open and closed positions by a motor that is mountedoutside the duct and connected to the vane shaft.

The aforesaid type of mechanical damper assembly is somewhat difficultto install in an existing duct. Installation requires the duct piece ofthe damper assembly to be spliced into an existing duct. This involvescutting a length from the existing duct and usually dismantling theexisting duct to enable such cutting and/or assembly of the duct piecebetween adjacent sections of the existing duct. This dismantling,cutting and reassembly of the duct work is time consuming and expensive.

Dampers also have been employed at room registers to control the flow ofair into a room. These dampers typically include a pivotally mounteddamper vane on the duct side of the register and an actuator lever whichprotrudes on the room side of the register for enabling manualadjustment of the damper vane. The dampers are wall or ceiling mountedover a duct opening or slip fitted in a duct opening provided in thefloor.

Also known are pneumatically actuated damper assemblies. Reference maybe had, for example, to U.S. Pat. No. 5,458,148 which discloses aself-contained unit comprising a support base for external mounting to aside of a duct and a damper vane mounted to the support base formovement between open and closed positions. The damper vane is locatedinwardly of the inner side of the support base for positioninginteriorly of the duct when the support base is mounted to the duct. Anactuator is mounted to the support base at the exterior side thereof andis operatively connected to the damper vane by a linkage for moving thedamper between the open and closed positions. The support base functionsto close an access opening in the side wall of the duct of sufficientsize to permit insertion of the damper vane therethrough.

SUMMARY OF THE INVENTION

The present invention provides a novel fluid flow control damperassembly that overcomes drawbacks associated with and/or improves uponprior art damper assemblies. Several embodiments of a damper assemblyaccording to the present invention are self-contained units that arecompact and relatively easy to install in existing ducts, including insituations where the outside surface of the duct is inaccessible.

A damper assembly according to one aspect of the present inventionincludes a damper vane and a pneumatic piston-cylinder actuatorconnected to the damper vane for moving the damper vane between an openposition and a closed position. The piston-cylinder actuator includes apiston and a cylinder that have cooperating nonelastomeric slidingsealing surfaces which provide essentially friction-free axial movementof the piston relative to the cylinder. The surfaces at the slidinginterface between the piston and the cylinder preferably are smooth andfunction to provide a substantially air tight seal, thereby eliminatingthe need for elastomeric seals that normally would introduce additionalfriction into the actuator.

The piston and/or the cylinder are formed of a material selected fromthe group including metal, powdered metal, resin, glass, and polymer.The piston and/or the cylinder also may have a friction-reducing coatingon the sealing surface thereof. Preferably, the piston-cylinder actuatorhas a powder-metal piston and a glass cylinder.

According to another aspect of the invention, the damper assemblyincludes a base, to which the damper vane and the piston-cylinderactuator are mounted, preferably on the same side of the base as thepiston-cylinder actuator. The actuator may be controllably connected toa pneumatic control system through a fitting at one end of the cylinder,and the fitting may have a threaded portion for mounting the actuatorwith respect to the base. The base may be configured for mounting to theexterior of a duct, in which case the piston-cylinder actuator may bemounted to the base with a nipple portion of the actuator fitting isdisposed at an exterior side of the base for connection to the pneumaticcontrol system. As a result, the piston actuator presents a minimalprofile outside the duct, which is advantageous in shallow clearancesituations, as well as providing an improved appearance.

The damper vane and the piston-cylinder actuator may be interconnectedby a drive rod. The drive rod may be mounted to transfer substantiallyonly axial forces from the actuator to the damper vane, for example, byusing ball joints to connect the ends of the drive rod to the dampervane and the actuator piston. This arrangement prevents lateral ortwisting forces from acting on the drive rod, which otherwise may causethe piston to bind in the cylinder. In addition, the actuator does nothave to be precisely aligned with the damper vane, thereby facilitatingmanufacturing and installation.

The piston-cylinder actuator also may have a cylindrical cap that fitsover an end of the cylinder to retain the piston in the cylinder. Thecap has an oversized opening for passage of the drive rod therethrough.Particularly if the cylinder is made of glass, a substantiallycylindrical protective boot may be used to substantially cover the outercircumferential surface of the cylinder. The boot preferably is made ofan elastomer, such as rubber, to protect the cylinder, particularlyprior to and during installation.

The damper vane also may include a plurality of damper vanes thatcooperate to close a cross-section of a duct when the damper vanes arein closed positions, and a plurality of piston actuators. Each pistonactuator is operatively connected to one of the damper vanes for openingand closing the damper vanes. The plurality of damper vanes may includea first damper vane that has an aperture, and a second damper vane thatcloses the aperture when both the first damper vane and the seconddamper vane are in their respective closed positions. The damper vaneslie in substantially parallel planes when both the damper vanes are intheir open positions to present a minimal profile to the airflow. Theplurality of damper vanes are used to supply different volumes of airthrough the damper assembly to mix fresh air with return air in varyingproportions, to provide a varied amount of conditioned air according tovaried requirements and/or to supply a constant but smaller amount ofairflow until a greater or full amount of air supply is needed.

The damper assembly may include a spring connected to the damper vanefor biasing the damper vane to a predetermined position. This feature isparticularly advantageous in the event of a power failure, toautomatically open or close the damper vane under the action of thespring.

According to another aspect of the invention, a damper assembly may havea frame, a damper vane mounted to the frame for pivotal movement betweenopen and closed positions, and an actuator for moving the damper vanebetween the open and closed positions. The frame has a cross-sectionalshape that approximates or generally corresponds to a cross-sectionalshape of a duct into which the damper assembly may be installed. Theframe preferably has a gasket or elastomeric member disposed on theexterior thereof or positioned about at least a portion of its outerperiphery to sealingly engage the frame with a duct wall. The actuatoris located within the confines of a projection of the frame in an axialdirection. Accordingly, the damper assembly may be inserted into an openend of a duct with the actuator disposed within the duct.

According to a further aspect of the invention, there is provided amethod of installing a damper assembly, the method including the stepsof removing a register from an outlet of a duct, inserting the damperassembly into a duct adjacent the outlet, and replacing the register.Thus, the simple method provided by the present invention may be used toretrofit a building where access to the ductwork is otherwiseunavailable.

According to another aspect of the present invention, a damper assemblymay include at least two duct wall-engaging members movable towards eachother against a biasing force. The duct wall-engaging members preferablyapply substantially equal forces against opposing walls of the duct tocenter and align the damper assembly in the duct. The duct wall-engagingmembers may apply unequal force against the duct wall to position thedamper assembly off-center as well. The duct wall-engaging members maybe formed by a leaf spring extending from lateral sides of the base, andmay have integral handle portions for easily grasping the damperassembly for insertion into a duct. Installation in a duct may beaffected by retracting the duct wall-engaging members against thebiasing force, inserting the damper assembly into the duct, andreleasing the duct wall-engaging members so that the biasing forcepresses the duct-wall engaging members against the duct walls to hold,align and preferably center the damper assembly in a fixed locationwithin the duct. A damper assembly having such wall-engaging members isparticularly advantageous when the external surface of the duct isinaccessible, permitting the damper assembly to be installed from an endof the duct, perhaps with just one hand. Automatic self-centering is aparticular advantage in a blind installation where the installer cannoteasily see into the duct. By automatically centering the damperassembly, the present invention also minimizes the chance that rigidportions of the damper vane would scrape against the duct wall as thedamper vane moves between its open and closed positions. The damperassembly is thus more likely to operate quietly.

The foregoing and other features of the invention are hereinafter fullydescribed and particularly pointed out in the claims, the followingdescription and annexed drawings setting forth in detail certainillustrative embodiments of the invention, these embodiments beingindicative, however, of but a few of the various ways in which theprinciples of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective side view of a damper assembly in accordancewith the present invention.

FIG. 2 is a partial cross-sectional side view of a duct including thedamper assembly of FIG. 1.

FIG. 3 is a rear view of the damper assembly and duct of FIG. 2.

FIG. 4 is an enlarged partial cross-sectional view of a damper assemblyin accordance with the present invention.

FIG. 5 is a partial sectional view of another damper assembly inaccordance with the present invention in a duct.

FIG. 6 is a partial sectional side view of another damper assembly inaccordance with the present invention in a duct.

FIG. 7 is a rear perspective view of another damper assembly inaccordance with the present invention.

FIG. 8 is cross-sectional side view of the damper assembly of FIG. 7.

FIG. 9 is a cross-sectional side view of yet another damper assembly inaccordance with the present invention.

FIG. 10 is a rear perspective view of still another damper assembly inaccordance with the present invention.

FIG. 11 is a cross-sectional side view of the damper assembly of FIG.10.

FIG. 12 is a cross-sectional side view of the damper assembly of FIG.10, with a smaller damper vane in an open position and a larger dampervane in a closed position.

FIG. 13 is a cross-sectional side view of the damper assembly of FIG. 10with both vanes in respective open positions.

FIG. 14 is a cross-sectional side view of another damper assembly inaccordance with the present invention.

FIG. 15 is a cross-sectional side view of the damper assembly of FIG. 14with a damper vane in an open position.

DETAILED DESCRIPTION

Referring initially to FIGS. 1-3, a damper assembly 20 in accordancewith the present invention includes a damper vane 22, a base 24 and apiston actuator 26.

The damper vane 22 generally is formed of rigid plate 28, such as ametal plate, and may also include a resilient member 30 that projectsbeyond the rigid plate to form a sealing flap along the peripheral edgeof the rigid plate 28. The damper vane generally has a shape thatapproximates the shape of a cross-section of a duct 32, such as arectangular shape or a disk or circular shape, and the sealing flap canmold to the contour of the duct to provide a positive seal even if theduct is dented or otherwise irregularly shaped, such as, for example, aduct having a raised seam. Consequently, when the damper vane is in itsclosed position, generally perpendicular to the axis of the duct, theperipheral sealing flap resiliently engages and seals against the innersurface of the duct to close the gap between the outer periphery of therigid plate and the inner surface of the duct.

The damper vane 22 is pivotally mounted to the distal end of a supportarm 34 that projects from one side of the base 24. For larger dampervanes, multiple support arms may be used to support the damper vane forrotation (see FIG. 10, for example, reference number 222). The dampervane is pivotally movable between a position generally parallel to thebase 24 (its open position) and a position generally perpendicular tothe base (its closed position). In the illustrated embodiment, thesupport arm is a rod to which the damper vane is pivotally connectedthrough a pivot mount 36 provided at the center of the damper vane.

The illustrated pivot mount 36 is a pivot block centrally secured to thedamper vane 22. The pivot block has a clevis pin 38 at one side thereoffor pivotally mounting the pivot block, and thus the damper vane, to thesupport rod 34. The damper vane pivots about an axis parallel to thedamper vane that passes through the clevis pin. The pivot block alsoincludes a drive pin 40 spaced from the clevis pin that provides a pointof attachment for a drive rod 42 interconnecting the damper vane 22 andthe piston actuator 26.

The piston actuator 26 is a pneumatically actuated piston-cylinderactuator mounted to the base by a bracket 43. The piston-cylinderactuator includes a piston 44 axially movable within a cylinder 46. Thepiston is sized and shaped to fit closely within the cylinder. Thepiston and cylinder have smooth surfaces at the sliding interfacetherebetween for relatively friction-free movement. The material for thecylinder is selected from the group including glass, resins andpolymers. In the illustrated embodiment, the cylinder is a glasscylinder (the cylinder may be formed of Pyrex® glass, for example)having an integral fitting 50 that closes one end of the cylinder. Thefitting has an outwardly projecting neck externally threaded tofacilitate securing the actuator to the bracket. The fitting also isprovided with a nipple 52 for connection to a pneumatic control system(not shown), as by means of a flexible tube 53. The control system mayfurther include a controller and a vacuum source or fluid pump toprovide vacuum/pressure control signals to the actuator to move thepiston 44.

The other end of the cylinder 46 opposite the fitting 50 is providedwith a cap 54 to retain the piston 44 in the cylinder. The cap may bemade of plastic, for example, and has an oversized opening 55 thereinfor passage of the drive rod 42 therethrough to connect to the piston.

The piston 44 preferably is a cylindrical metal piston that closely fitswithin the cylinder 46 for smooth, low friction, sliding movement. Thepiston may be made of a material selected from the group includingmetal, powder metal, glass, resins and polymers. The piston may have acoating of a friction-reducing material such as a Teflon®(polytetrafluoroethylene) coating or a dry lubricant formed from carbonpowder, for example, at the surface thereof. The outer surface of thepiston and the inner surface of the cylinder are smooth to provide aninterface that functions as an air tight seal, thereby eliminating theneed for elastomeric seals or the like that normally would introduceadditional friction into the system.

The piston 44 is connected to the damper vane 22 by the drive rod 42.The drive rod is connected between the piston and the damper vane suchthat linear movement of the piston effects pivotal movement of thedamper vane. The drive rod is connected to the piston by a ball joint58. A loop 59 in the opposite end of the drive rod passes around thedrive pin 40 in the pivot block 36. This arrangement allows the drivepin to rotate within the loop while transferring axial loads through thedrive rod. Slop or lateral space along the drive pin between portions ofthe pivot block relative to the width of the loop in the drive rodcompensates for misalignment between the pivot block 36 and the actuator26, allowing the actuator to pivot the damper vane 22 withouttransferring lateral forces through the drive rod that could cause thepiston to bind in the cylinder 46. Alternatively, ball joints may beprovided at both ends of the drive rod to preclude side loads andtwisting moments from acting on the drive rod to bind the piston in thecylinder.

An enlarged view of an actuator 60 that is substantially the same as theactuator 26, is shown in FIG. 4. The actuator includes a glass cylinder62 having a fitting 64 that closes one end. The fitting has an extension66 that includes a nipple 68 that is connected to a control system (notshown) via a tube 70. The extension has a threaded portion that enablesa nut 72 to secure the actuator to a bracket 74 that is connected to abase 76.

A support rod 78 fixed at one end of the base 76 supports at itsopposite end a damper vane 80 through a pivot block 82. The pivot blockincludes a drive pin 84 around which an end of a drive rod 86 is looped.The opposite end of the drive rod is connected to the piston with a balljoint 90. The piston is movably mounted within the cylinder and retainedtherein by a cap 92 at an end of the cylinder opposite the fitting.

The actuator also includes a protective boot 94 covering the outersurface of the cylinder, which boot may be made of rubber or otherelastomeric or suitable material. Both the boot and the cap haveconcentric openings 95, 96, respectively, for passage of the drive rodtherethrough. In the illustrated embodiment, the damper vane is in itsopen position.

Returning to FIGS. 1-3, the damper assembly 20 further includes at leasttwo oppositely directed wall-engaging members 100 movable towards eachother against a biasing force to insert and then hold the damperassembly in the duct 32. The wall-engaging members can be moved towardeach other and held in a retracted position for inserting or removingthe damper assembly from a duct. When the wall-engaging members arereleased, under the influence of the bias force they move toward a biasposition and press against the walls of the duct. The wall-engagingmembers hold and automatically align and center the damper assemblywithin the duct.

The wall-engaging members 100 in the illustrated embodiment areresilient wings formed by a leaf spring connected to the base 24. In theillustrated embodiment, the leaf spring includes handles (finger grips)102 for holding the leaf spring in the retracted position for insertioninto the duct 32. The handles preferably are integrally formed at thedistal ends of the leaf spring. Although the illustrated embodiment usesa leaf spring, other wall-engaging members may be used to secure thedamper assembly within a duct, such as a different spring arrangement, ascissor-jack arrangement, or a deformable expansion device biased to anexpanded position.

The duct wall-engaging members 100 preferably apply substantially equalforce against opposing walls of the duct 32 to center and align thedamper assembly 20 in the duct. The wall-engaging members may applyunequal force against the duct wall to position the damper assemblyoff-center as well. A damper assembly having such wall-engaging membersis particularly advantageous when the external surface of the duct isinaccessible, permitting the damper assembly to be installed from an endof the duct, perhaps with just one hand. The automatic positioningprovided by the wall-engaging members is particularly advantageous in ablind installation where the installer cannot easily see into the duct.In addition, the automatic positioning provided by the wall-engagingmembers minimizes the chance that the rigid portions of the damper vanewould scrape against the duct wall as it moves, thereby providingfurther assurances that the damper assembly would generate minimal noisein operation.

Referring now to FIG. 5, a damper assembly 110 is shown that isinstalled through an opening 112 in the wall of a duct 114. Such amethod of installation is shown and described in commonly owned U.S.Pat. No. 5,458,148, the entire disclosure of which is herebyincorporated herein by reference. Naturally, this type of installationrequires access to an outside surface of the duct.

The damper assembly 110 includes a base 116 that covers the opening 112in the duct 114 and is secured to the outside of the duct, using, forexample, mechanical fasteners (e.g., sheet metal screws), an adhesive,or even a magnet. The base has a curved shape that approximates thecontour of the outer surface of the duct. An actuator 118 and a dampervane 120 are supported by the base such that the damper vane and theactuator are substantially contained within the duct. The actuator 118is substantially the same as the actuator 26 described above, althoughrather than being supported on a bracket, the actuator 118 is mounteddirectly to the base 116 such that the nipple 122 extends outside theduct for connection to a control assembly (not shown) through a supplyline 124. In the illustrated embodiment, the actuator is perpendicularto the base and the damper vane is in its closed position. The openposition of the damper vane, and the corresponding position of thepiston actuator, are shown in phantom lines. In this configuration thepiston actuator presents a minimal profile outside the duct, which isadvantageous in shallow clearance situations, as well as providing animproved appearance.

Referring now to FIG. 6, another damper assembly 130 is shown that isinstalled through an opening 132 in the wall of a duct 134. Unless thedifference is specifically identified, the damper assembly 130 issubstantially identical to the damper assembly 110 shown in FIG. 5. Asin FIG. 5, the damper assembly 130 is shown in a closed condition, withthe position of the damper vane 136 and the other components, when thedamper vane is in an open position, being shown in phantom lines. Thedamper assembly includes a base 137 that is attached to the outside ofthe duct, a damper vane 136 supported within the duct by the base, andan actuator 138 operatively connected to the damper vane through a driverod 140 and a pair of ball joints 142. The actuator includes a piston144 movable within a cylinder 146.

Although the range of travel of the piston 144 within the cylinder 146may be calibrated to provide the desired movement of the damper vane136, it may be difficult or impossible to maintain accurate movement ofthe damper vane over time. Consequently, positive stops are provided tofacilitate consistently moving the damper vane between the open positionand the closed position. In the illustrated embodiment, the actuator 138is mounted to the base 137 at an angled orientation by a bracket 148.The bracket 148 is substantially similar to the bracket 43 describedabove with respect to FIGS. 1-3. However, in the embodiment shown inFIG. 6 the bracket 148 extends past the point where the actuator ismounted to provide a stop that engages the damper vane 136 when thedamper vane is in the open position. The bracket/stop positively locatesthe damper vane in its open position. The support rod 150 and/or aportion of the base may provide a similar function when the damper vaneis in the closed position (as shown, for example, in FIG. 2).

In addition, the pivot block 82 may be configured to provide positivestops for limiting movement of the damper vane 136 beyond its open orclosed positions. For example, as shown in FIG. 4, the pivot block 82has a stop surface 152 that abuts a side of the support rod 78 when thedamper vane 80 is pivoted to its open position.

Moving on to FIGS. 7 and 8, another a damper assembly 160 is shown. Thedamper assembly may be inserted into a duct (not shown) adjacent theduct outlet, generally into a part of the duct referred to as the boot,where the damper assembly may be held in the duct by gravity or with afriction fit. The base of the damper assembly is in the form of a frame162 having side walls 164 configured to approximate the cross-sectionalshape of a duct. The frame may include mounting extensions or mountingtabs 166 extending from the side walls for securing the damper assemblywithin the duct, for example, using mechanical fasteners installedinside the duct from the duct outlet. The frame also has a gasket 168circumferentially attached to at least a portion of the outercircumference of the frame to sealingly engage the inner circumferenceof the duct and to fill any voids between the damper and the duct. Thegasket may have a teardrop cross-sectional shape to provide an improvedfit between the outside of the frame and the duct.

Inside the frame 162, the damper vane 170 is a metal plate pivotallysupported for rotation between open and closed positions, the openposition being shown in phantom in FIG. 8. The frame includes stopportions 172 that cooperate to define the closed position. The stopportions include a sealing material 171 that cooperates with the dampervane to seal the opening formed by the frame when the damper vane is ina closed position. Alternatively, the sealing material may be applied tothe damper vane so as to contact the stops when the damper vane moves tothe closed position.

An actuator 176, substantially similar to the actuator 26 describedabove, is mounted to the frame through a bracket 178 and moves thedamper vane between its open and closed positions. The actuator isconnected to the damper vane through a slot 180 in the damper vane and adrive pin 182 mounted between a pair of bent tabs 184 extending from thedamper vane on a side opposite the actuator. The actuator may becentrally mounted or offset mounted, for example, for use with offsetend boots. However, the actuator is mounted within the confines of aprojection of the frame 162 in an axial direction (parallel to the axisof the duct) so that the entire damper assembly fits within a duct. Thesupply line for the control assembly, such as pneumatic tubing 186, canbe fished through the duct to the control assembly or to a locationwhere it can conveniently be brought out of the duct.

This type of damper assembly is particularly advantageous where the onlyaccess to the duct is through a duct outlet. This is made possible bythe small size of the actuator needed to operate the damper vane.Furthermore, the actuator can be located within the duct without undulycompromising the cross-sectional area of the duct available for fluidflow. Thus, the present invention provides for the ability to retrofitbuildings with inaccessible or enclosed ductwork to provide zone-basedheating or cooling that heretofore was impractical.

Another damper assembly 190, shown in FIG. 9, is substantially the sameas the damper assembly 160 shown in FIGS. 7 and 8, except as otherwisenoted. The damper assembly 190 includes multiple damper vanes 192 and194 with each vane having its own actuator 196, 198, respectively. Theuse of multiple damper vanes is particularly advantageous in largerducts where a larger damper vane would require greater clearance betweenthe damper assembly and the duct outlet. The open positions of thedamper vanes are shown in phantom lines. One or more of the damper vanesmay include a sealing material 200 that separates the damper vanes whenan adjacent damper vane is in its closed position. The sealing materialnot only helps to create a tight seal but also minimizes or eliminatesvibration and noise at the junction of the damper vanes. Themultiple-vane design also allows for additional control or regulation ofthe airflow through the damper assembly, particularly infresh-air/economizer HVAC systems.

The supply lines 202 from the controller (not shown) to the actuatorsmay include one or more one-way restrictor valves 204 that restricts thefluid control signal so that the damper vanes move to the closedposition in a sequential manner, with the damper vane having morerestrictor valves limiting the control signal and causing that dampervane 198 to move more slowly, thereby avoiding interference at theoverlap or junction between the damper vanes as they move to theirclosed positions.

An alternative multiple vane damper assembly 210 is shown in FIGS.10-15. Referring initially to FIGS. 10-13, the damper assembly 210includes two damper vanes: a first or larger damper vane 212 and asecond or smaller “window” damper vane 214. The larger damper vane 212includes a window or aperture 216 therein and the smaller damper vane214 seals the aperture when both damper vanes are in their closedpositions, as shown in FIG. 10. The damper vanes are formed of rigidplates with gaskets 218, 220 that form sealing flaps about the outerperiphery of the larger and smaller damper vanes, respectively. In theillustrated embodiment, the larger damper vane does not include a gasketwithin the aperture, although one could be provided. The smaller dampervane can be moved to its open position independently of the largerdamper vane to provide a smaller airflow, as shown in FIG. 12. When bothdamper vanes are in their open positions, as shown in FIG. 13, thedamper vanes will lie in the same or substantially parallel planes.

In the illustrated embodiment, each damper vane 212, 214 is supportedfor pivotal movement on three sets of support rods 222 and pivot blocks224, with the central pivot block for each damper vane being connectedto a drive rod 226 that is connected to a diaphragm actuator 228. Thedamper vanes are supported on a first side 230 of a base 232, and theactuators are connected to a second side 234 of the base opposite thefirst side. The illustrated base has a sealing material 236 about atleast a portion of the circumference of the first side of the base. Thedamper assembly 210 can be installed through an opening 237 in the sideof a duct 239, and the sealing material helps seal the opening when thebase is secured to an external surface of the duct. The illustrateddamper assembly includes diaphragm actuators 228, such as thosedisclosed in the aforementioned U.S. Pat. No. 5,458,148. However, theabove-described pneumatic piston-cylinder actuators could be used intheir place, particularly when access to the outer surface of the ductis limited or unavailable.

Although only two damper vanes are shown in the illustrated embodiment,any number of damper vanes could be used to supply different volumes ofair through the damper assembly to mix fresh air with return air invarying proportions, to bring in a large amount of fresh air for coolingwhen outdoor air temperatures permit, to provide a varied amount ofconditioned air according to varied requirements and/or to supply aconstant but smaller amount of airflow until a greater or full amount ofair supply is needed. Another advantage is that by opening only aminimal amount of damper vanes, noise created by a large volume of airdirected toward a relatively small outlet is minimized or eliminated.

The damper assembly 211 shown in FIGS. 14 and 15 is the same as thedamper assembly 210 in FIGS. 10-13 except that the damper assembly 211further includes a spring 240 connected to the drive rod 226 between awasher 242 mounted to the drive rod and the base 232 to bias the dampervane 214 toward either the open position or the closed position: in thiscase, the closed position. This feature is particularly advantageous inthe event of a power failure, such that removal of a control signal, inthe form of a vacuum or pressurized fluid applied to the actuator 228,provides automatic closure of the damper vane under the action of thespring or other biasing device. The spring may also be used as 8 a powerassist to reduce the power required to open or close the damper vane. Inaddition, a magnet could be used to hold the damper vane in either anopen or a closed position, wherein in the event of a power failure, thedamper vane would remain in that position.

Although the invention has been shown and described with respect tocertain illustrative embodiments, equivalent alterations andmodifications will occur to others skilled in the art upon reading andunderstanding this specification and the annexed drawings. In particularregard to the various functions performed by the above describedintegers (components, assemblies, devices, compositions, etc.), theterms (including a reference to a “means”) used to describe suchintegers are intended to correspond, unless otherwise indicated, to anyinteger which performs the specified function of the described integer(i.e., that is functionally equivalent), even though not structurallyequivalent to the disclosed structure which performs the function in theherein illustrated exemplary embodiment or embodiments of the invention.In addition, while a particular feature of the invention may have beendescribed above with respect to only one of several illustratedembodiments, such feature may be combined with one or more otherfeatures of the other embodiments, as may be desired and advantageousfor any given or particular application.

What is claimed is:
 1. A damper assembly comprising a framecircumscribing an opening and configured for Insertion into a duct; adamper vane mounted to the frame for movement between an open positionand a closed position, and a piston-cylinder actuator connected betweenthe frame and the damper vane for moving the damper vane between theopen and closed positions; the piston-cylinder actuator including apiston and a cylinder that have cooperating nonelastomeric slidingsealing surfaces which provide essentially friction-free axial movementof the piston relative to the cylinder.
 2. A damper assembly as setforth in claim 1, wherein the cooperating sealing surfaces include asubstantially continuous inner circumferential surface of the cylinder,and a substantially continuous outer circumferential surface of thepiston.
 3. A damper assembly as set forth in claim 1, wherein the pistonis formed of a material selected from the group including metal,powdered metal, resin, glass, and polymer.
 4. A damper assembly as setforth in claim 3, wherein the sealing surface of the piston has afriction-reducing coating.
 5. A damper assembly as set forth in claim 1,wherein the cylinder is formed of a material selected from the groupincluding metal, powdered metal, resin, glass, and polymer.
 6. A damperassembly as set forth in claim 5, wherein the sealing surface of thecylinder has a friction-reducing coating.
 7. A damper assembly as setforth in claim 1, wherein the piston-cylinder actuator is a pneumaticpiston-cylinder actuator.
 8. A damper assembly as set forth in claim 7,wherein the piston-cylinder actuator includes a fitting at one end ofthe cylinder to connect the actuator to a pneumatic control system.
 9. Adamper assembly as set forth in claim 1, further comprising a basemounted to the frame to support the piston-cylinder actuator.
 10. Adamper assembly as set forth in claim 9, wherein the piston-cylinderactuator includes a fitting at one end of the cylinder that has athreaded portion for mounting the actuator relative to the base.
 11. Adamper assembly as set forth in claim 9, wherein the piston-cylinderactuator is mounted substantially perpendicularly to the base.
 12. Adamper assembly as set forth in claim 9, wherein the damper vane and thepiston-cylinder actuator are mounted on the same side of the base.
 13. Adamper assembly as set forth in claim 12, further comprising a stopextending from the base for positively defining the open position or theclosed position of the damper vane.
 14. A damper assembly as set forthin claim 1, wherein the damper vane and the piston-cylinder actuator areinterconnected by a drive rod.
 15. A damper assembly as set forth inclaim 14, wherein the drive rod is mounted to transfer substantiallyonly axial forces between the actuator and the damper vane.
 16. A damperassembly as set forth in claim 1, further comprising a substantiallycylindrical protective boot that substantially covers the outercircumferential surface of the cylinder.
 17. A damper assembly as setforth in claim 1, further comprising a frame having side walls thatapproximate a cross-sectional shape of a duct.
 18. A damper assembly asset forth in claim 17, wherein the frame supports the damper vane forpivotal movement about an axis.
 19. A damper assembly comprising a framehaving side walls that approximate a cross-sectional shape of a duct, agasket positioned about at least a portion of an outer circumference ofthe frame, a damper vane movable between an open position and a closedposition, and a piston-cylinder actuator connected to the damper vanefor moving the damper vane between the open and closed positions; thepiston-cylinder actuator including a piston and a cylinder that havecooperating nonelastomeric sliding sealing surfaces which provideessentially friction-free axial movement of the piston relative to thecylinder.
 20. A damper assembly as set forth in claim 1, wherein thedamper vane includes a plurality of damper vanes that cooperate to closea cross-section of a duct when the damper vanes are in closed positions.21. A damper assembly as set forth in claim 20, wherein the pistonactuator includes a plurality of piston actuators, each piston actuatoroperatively connected to one of the plurality of damper vanes.
 22. Adamper assembly as set forth in claim 20, wherein the plurality ofdamper vanes includes a first damper vane that has an aperture and asecond damper vane that closes the aperture when both the first dampervane and the second damper vane are in their respective closedpositions.
 23. A damper assembly as set forth in claim 22, wherein thefirst damper vane and the second damper vane lie in substantiallyparallel planes when both the first damper vane and the second dampervane are in the open positions.
 24. A duct section having a damperassembly therein as set forth in claim
 1. 25. A damper assemblycomprising a frame having a cross-sectional shape generallycorresponding to a cross-sectional shape of a duct, at least one dampervane mounted to the frame for pivotal movement between an open positionand a closed position, at least one actuator connected to the at leastone damper vane to controllably move the at least one damper vanebetween the open and closed positions, the at least one actuator locatedwithin the confines of a projection of the frame in an axial direction,and an elastomeric member disposed on the exterior of the frame tosealingly engage the frame with a duct wall.
 26. A damper assembly asset forth in claim 25, wherein the elastomeric member surrounds theperiphery of the frame.
 27. A method of installing a damper assembly ina duct, comprising the steps of: providing a damper assembly as setforth in claim 1, removing a register from a duct outlet, inserting thedamper assembly into a duct adjacent the outlet, and replacing theregister.